Clinical electromyographic (EMG) techniques are used in the differential diagnosis of various neurogenic and myogenic disorders. Needle EMG and fine wire recording techniques are also used for basic motor control research studies to better understand motor unit (MU) recruitment and discharge properties. Under controlled conditions (e.g., constant posture, slowly force-varying, lower-force contractions), these techniques can identify individual MUs in the vicinity of the recording electrode. Limitations of these invasive techniques include some risk of infection, possible subject/patient discomfort, and the need to record from a small portion of the muscle. Thus, recordings must be obtained at numerous muscle locations to obtain representative MU discharge behavior samples. Invasive electrodes are unsuitable for monitoring the spread of electrical excitation along the muscle, which can reveal dynamic properties of the muscle fiber membrane. Recently, tightly-spaced surface electrode arrays have demonstrated sufficient spatial resolution to monitor the activity of individual MUs. Our overall objective is to develop novel apparatus and analysis methods to permit MU action potential detection and tracking in noninvasive surface EMG arrays. This new technique would relax several restrictions of invasive techniques by allowing simultaneous recording from numerous muscle areas, as well as numerous sites along the muscle fiber membrane. This technique would have the additional advantage of providing estimates of the muscle fiber conduction velocity (MFCV) of muscle fibers comprising individual MUs. The specific aims of this project are to (a) develop a novel surface electrode array recording system that provides both high-fidelity and flexible electrode montage selection (existing systems are either high-fidelity or flexible, but not both), (b) use the array to develop noninvasive methods to measure standard discharge properties along with MFCV (a MU parameter that is not presently measured clinically) and MU size (noninvasive macro-EMG), and (c) validate standard discharge information versus simultaneously measured needle EMG in a pilot experiment comparing MU properties between younger and aged healthy subjects over a range of forces. The array recording system will identify many of the MUs simultaneously obtained from the indwelling electrodes. In the experimental portion of this work, we hypothesize that (a) during lower force levels, the active MUs will tend to have slower firing rates, smaller size (as measured by macroEMG) and slower MFCV than those MUs that are recruited at higher force levels and (b) compared to aged subjects, younger subjects will tend to have higher firing rates, faster MFCVs, and a greater dispersion of MFCVs. The long-term goal is to develop a non-invasive system for recording a complete set of MU discharge characteristics. This system should lead to applications in areas such as ergonomics, clinical EMG, and basic motor control research.